The invention relates generally to remote sensing of electromagnetic fields, and more particularly to sensor systems using electro-optic sensors.
Lithium niobate devices have been used for electromagnetic field sensing. Several concepts have been proposed to increase the sensitivity of such devices while others have focused on increasing the frequency bandwidth of the devices. Electromagnetic field sensing applications are wide varying. Sensor designs generally attempt to meet one or more of the following criteria: (i) minimal perturbance of the electromagnetic field; (ii) well-characterized frequency response over a broad frequency range; (iii) large dynamic range; and (iv) small size to allow spatially resolved measurements in small regions. However, prior sensor designs have not been known to be able to discriminate against environmental variables such as temperature, vibration, pressure and humidity.
Lithium niobate integrated optic modulators are suitable transducers for field sensing applications. Since such devices are both powered and interrogated over optical fibers, the detecting and measuring hardware can be located remotely from the sensing site so that there is minimal perturbation of the fields being measured. Lithium niobate integrated optic modulators are compact, lightweight, and can have high potential bandwidth and high voltage sensitivity.
Known lumped element electrode structures for sensing applications are limited in bandwidth to about 1 or 2 GHz. In a field sensing application, the electrodes on the lithium niobate devices are typically connected to antennas. Broadband matching of electrode and antenna impedances is difficult. For many antenna designs, the electrode transmission line cannot be terminated in its characteristic impedance without severely loading the antenna. One way to increase bandwidth is to use short electrodes with very small capacitance so that even at the highest desired frequency the antenna is not loaded. However, the voltage sensitivity of the sensor device goes down in proportion to decreased electrode length.